圆周角是圆中一类特殊的角,正确理解圆周角的概念,深刻认识圆周角定理,切实掌握圆周角定理的应用,是学好圆周角的关键,也是进一步学好圆的相关知识的基础.例1如图1,图1⊙O是等腰三角形ABC的外接圆,AB=AC,∠A=45°,BD为⊙O的直径,BD=2 2%姨,连接CD,则∠D=______,BC=______.【解析】由“同弧所对的圆周角相等”可知,∠D=∠A=45°;又BD为⊙O的直径,所以∠BCD=90°;又BD=2 2%姨,所以BC=CD=2. Circumferential angle is a special kind of circle. Understanding the concept of circular angle correctly, knowing the circular angle theorem well and applying the circular angle theorem is the key to learning the circular angle, which is also the basis for further learning the related knowledge of circle. 1 As shown in Figure 1, Figure 1 ⊙O is the circumcircle of the isosceles triangle ABC, AB = AC, ∠A = 45 °, BD is the diameter of ⊙ O, BD = 2% aunt, connecting CD, then ∠D = ______ , BC = ______. [Analysis] from the same angle of the same arc ", we can see that ∠D = ∠A = 45 °; and BD is the diameter of ⊙O, so ∠BCD = 90 °; BD = 2 2% Aunt, so BC = CD = 2.